The invention relates to a circuit arrangement for operating a high-pressure sodium lamp with a controllable colour temperature T.sub.c in which a power P is supplied to the lamp through current pulses having a current value I and a duty cycle D, whereby a change in the colour temperature T.sub.c is achieved through a change .DELTA.D of the duty cycle.
A conventional circuit arrangement, such as disclosed in U.S. Pat. No. 4,137,484 varies the colour temperature T.sub.c of light radiated by a high pressure to sodium lamp between approximately 2,000 K. and approximately 2,900 K., provided the power P supplied the lamp is constant. In other words, a change in the duty cycle .DELTA.D must be accompanied by a corresponding change in the current value .DELTA.I, where the relation .DELTA.I/.DELTA.D&lt;0 is satisfied. Colour temperature variation from 2,000 K. to 2,900 K. results in a decrease of approximately 20% in the luminous flux radiated by the lamp. Such a decrease in the luminous flux although perceptible by the human eye is too small to effectively serve as a dimmer for the lamp. For dimming purposes a practical dimming facility the luminous flux should controllably range between a ratio of at least 1:4, preferably 1:5. Conventional circuit arrangements, however, cannot provide such ratios without current rising to an unacceptable level. There is also a risk of colour temperature T.sub.c falling well below 2,000 K. for high luminous flux values. A colour temperature of 2,000 K. or less, is generally less desirable for applications other than public lighting.